1. Field of the Invention
The invention relates to an apparatus for determining rotational position of a rotatable element, especially a rotating shaft in a motor vehicle, without contacting it and, more particularly, to an apparatus for determining rotational position that includes a sensor device having Hall or AMR sensor elements for sensing a magnetic field of magnetic field strength (B) generated by or influenced by the rotational position of the rotatable element and for producing output signals according to the magnetic field and thus the rotational position of the rotatable element.
2. Prior Art
An apparatus of this type, with which detection of the angle of rotation can be carried out, is known from EP-O 217 478 B1, for example. Here the magnetic field lines issuing from a rotating magnet are detected and evaluated by an angle sensor, which is formed from two parallel thin films of magnetically soft material. The magnetic films in the angle sensor are mounted such that in the plane of the field lines power supply connections and voltage measuring connections, respectively, are staggered by 45.degree. and connected to the two films. Due to resistance anisotropy in the films the result is a sensor signal dependent on the direction of the angular position of the field lines, a signal that can be processed in an evaluation circuit. In this known configuration, however, angle determination can take place only in the range of 180.degree., which results in higher cost for proper detection of the angular position.
Sampling of incremental structures on gear wheels made of ferromagnetic material is also known, where sampling is carried out using contactless sensor configurations, for ignition control in internal combustion engines or for speed detection for anti-lock systems, for example. An article in VDI-Bericht, No. 509 (VDI-Verlag 1984), Pages 263 to 268, entitled "Neue, alternative Losungen fur Drehzahlsensoren im Kraftfahrzeug auf magnetoresistiver Basis" New Alternative Solutions for Magnetoresistive-Based Speed Sensors in Motor Vehicles!, for example, describes how sampling of incremental structures on rotating shafts or gear wheels in a manner that is especially simple and particularly insensitive to air-gap fluctuations can be carried out using magnetic tangential probes as sensors. This is possible because the sign of the tangential component of a permanent magnet located in the sensor is not a function of the size of the air gap between the sensor and the rotatable elements. A change in this sign can only be effected by a further rotor-like movement of the shaft or gear wheel, by which means an incremental change in the angle of rotation of a gear wheel can be detected.
The known publication cited above also describes the use of sensors that utilize the "Hall effect" for sampling a change in a magnetic field on page 264 in section 3.3. These Hall sensors, the design of which is known, utilize deflection of a current flowing between two connection poles brought about by a magnetic field positioned perpendicular to it. In this case a charge carrier displacement in the direction of the magnetic field lines results in the development of "Hall voltage," which can be tapped at right angles to the current flow direction. Utilizing the properties of Hall structures in semiconductor materials that are optimum for this purpose, the current flow direction in this Hall structure generally runs coplanarly in the plane of a "semi-conductor wafer," for example a silicon semiconductor chip.
The tangential field detection necessary for sampling on rotatable elements is in this case disadvantageous, since as a result of the unavoidable tangential lengthening of the conventional type of Hall sensor, a considerable air gap between the rotatable element and the sensor results (approximately 2 mm to 4 mm) and the effect of the field lines running perpendicular to the wafer plane is sharply reduced. Since the field strength decreases exponentially in relation to the distance between the rotatable element and the sensor, the measurement effect that can be evaluated becomes very small in this case. In order to detect a signal that is not a function of the air gap using the known method, differential detection of the radially oriented field must be carried out with two Hall sensors that may also need to be adjusted, especially to the increment widths (tooth spacing in gear wheels). Use of materials having a higher sensitivity oriented in a different direction such as magnetoresistive thin-film sensors, permalloy sensors or even "pseudo-Hall sensors" as an alternative to silicon semiconductor Hall sensors is possible, but the manufacturing technology for these materials is expensive, particularly if they are to be connected to an amplifier circuit or integrated with it.
In addition, Hall sensors are known (from Sensors and Materials, 5.2 (1993) 091-101, MYU, Tokyo, the article "Simulation, Design and Fabrication of a Vertical Hall Device for Two-Dimensional Magnetic Field Sensing" by M. Parajape, Lg. Ristic and W. Allegretto) in which the Hall structure extends perpendicular to the wafer surface to the depth of a silicon chip. Thus in this case detection of the tangentially oriented field is possible without increasing the air gap to an intolerable degree.
These Hall sensors, as known from the second publication, require a small degree of expansion and consequently a very small air gap, which consists essentially of only the wafer thickness (approximately 400 .mu.m) plus a protective coating. Detection of components of a two-dimensional magnetic field using two Hall sensors that are offset by 90.degree. is also known from this publication.